Typically, multiple frequency bands are assigned to an operator of a radio communication system, and the respective operators use the assigned frequency bands (carriers) to provide radio communication services to user equipments (UEs) such as mobile phones and smartphones. In order to utilize these multiple frequency bands efficiently, the operators control to distribute the user equipments over the frequency bands (band balancing control or inter-frequency balancing control).
In the example as illustrated in FIG. 1, two frequency bands 1.5 GHz and 2 GHz are assigned to a certain operator, and the certain operator uses these two frequency bands to provide radio communication services to the user equipments. For example, in the case where the many user equipments reside in the frequency band of 1.5 GHz whereas the cell for the frequency band of 2 GHz is relatively sparse as illustrated in the left side in FIG. 1, a base station or a band balancing controller performs band balancing control or inter-frequency balancing control to cause the user equipments using the cell for the frequency band of 1.5 GHz to migrate to the cell for the frequency band of 2 GHz so as to achieve uniform amounts of traffic over these two carriers, as illustrated in the right side in FIG. 1.
For example, after the user equipment has entered an RRC_CONNECTED state in a camped cell at outgoing and incoming calls, the base station or the band balancing controller causes the user equipment to measure RSRQ (Reference Signal Received Quality) in the cells for the two frequency bands 1.5 GHz and 2 GHz and report the measured RSRQ. Based on the RSRQ for the two cells reported from the user equipment and congestion status, the base station or the band balancing controller causes the user equipment to perform handover to the cell for an appropriate one of the frequency bands. Alternatively, the base station or the band balancing controller causes the user equipment to report the RSRQ at terminating calls and migrate to the cell for the appropriate frequency band by an RRC Connection Release based on the reported RSRQ and the congestion status for the respective cells.
See 3GPP TS 36.300 V12.0.0 (2013-12), 3GPP TS 36.331 V12.0.0 (2013-12) and 3GPP TS 36.133 V12.2.0 (2013-12) in detail, for example.